Nanodiamond (ND) also referred to as ultrananocrystalline diamond or ultradispersed diamond (UDD) is a unique nanomaterial which can easily be produced in hundreds of kilograms by detonation synthesis.
Detonation nanodiamonds (ND) were first synthesized by researchers from the USSR in 1963 by explosive decomposition of high-explosive mixtures with negative oxygen balance in a non-oxidizing medium. A typical explosive mixture is a mixture of trinitrotoluene (TNT) and hexogen (RDX) and a preferred weight ratio of TNT/RDX is 40/60.
As a result of the detonation synthesis a diamond-bearing soot also referred to as detonation blend is obtained. This blend comprises nanodiamond particles, which typically have an average particle size of about 2 to 8 nm, and different kinds of non-diamond carbon contaminated by metals and metal oxide particles coming from the material of the detonation chamber. The content of nanodiamonds in the detonation blend is typically between 30 and 75% by weight.
The nanodiamond-containing blends obtained from the detonation contain same hard agglomerates, typically having a diameter of above 1 mm. Such agglomerates are difficult to break. Additionally the particle size distribution of the blend is very broad.
The diamond carbon comprises sp3 carbon and the non-diamond carbon mainly comprises sp2 carbon species, for example carbon onion, carbon fullerene shell, amorphous carbon, graphitic carbon or any combination thereof.
There are number of processes for the purification of the detonation blends. The purification stage is considered to be the most complicated and expensive stage in the production of nanodiamonds.
For isolating the end diamond-bearing product, a complex of chemical operations directed at either dissolving or gasifying the impurities present in the material are used. The impurities, as a rule, are of two kinds: non-carbon (metal oxides, salts etc.) and nondiamond forms of carbon (graphite, black, amorphous carbon).
Chemical purification techniques are based on the different stability of the diamond and non-diamond forms of carbon to oxidants. Liquid-phase oxidants offer an advantage over gas or solid systems, because they allow one to obtain higher reactant concentrations in the reaction zone and, therefore, provide high reaction rates.
In the recent years nanodiamonds have received more and more attention due to several existing applications within electroplating (both electrolytic and electroless), polishing, various polymer mechanical and thermal composites, CVD-seeding, oils and lubricants additives as well as possible new applications such as luminescence imaging, drug delivery, quantum engineering etc.
The fact that the available nanodiamond materials possess a variety of various surface functions and thus agglomeration (from several hundreds of nanometers to several microns), is effectively limiting their use in industrial applications. As applying agglomerating nanodiamond grades, very high filler loadings are typically required, making their cost efficient use impossible in most of the applications today. Moreover, nanodiamond agglomeration is effectively limiting or prohibiting the optimization of various application end product technical properties. Agglomeration is making it impossible to use nanodiamonds in applications wherein the product optical properties have to be retained; agglomeration is causing scratching in polishing and fine-polishing applications; agglomeration can have a direct adverse effect on polymer composite mechanical properties; agglomeration in an electroplating electrolyte or electroless deposition chemicals (due to non-optimal nanodiamond zeta potential as in relation to electrolyte pH regime) makes their usage completely impossible for manufacturing mechanically improved metal coatings; agglomeration is effectively prohibiting nanodiamond usage as a drug carrier material; agglomeration is having an adverse effect on CVD produced diamond film quality etc.
Cost efficient and technologically optimized usage of nanodiamond materials both in their powder, suspension and dispersion form can only be achieved if nanodiamonds are substantially mono-functionalized type and have thus, depending on the type of surface modification, the highest possible affinity to various solvents and polymer, metal or ceramic materials. This substantial mono-functionalization should also accomplish the production of single digit nanodiamond dispersions (dispersions wherein nanodiamonds are essentially in their primary particle, agglomerate free form). Such a substantially mono-functionalized nanodiamond possesses, depending on the type of surface functionalization, either a highly positive or negative zeta potential value.
The significance of zeta potential is that its value can be related to the stability of colloidal dispersions. The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles in dispersion. For molecules and particles that are small enough, a high zeta potential will confer stability, i.e., the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. So, colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate. If the zeta potential is 0 to ±5 mV, the colloid is subjected to rapid coagulation or flocculation. Zeta potential values ranging from ±10 mV to ±30 mV indicate incipient instability of the colloid (dispersion), values ranging from ±30 mV to ±40 mV indicate moderate stability, values ranging from ±40 mV to ±60 mV good stability as excellent stability is reached only with zeta potentials more than ±60 mV.
In order to be technologically viable and cost efficient, nanodiamond single digit dispersions should possess high nanodiamond concentrations. Preferably, nanodiamond concentrations should exceed 2 wt-%. If the concentration is too low, too much of excess solvent has to be added into application processes, having an adverse effect on metal ion and other additive concentration within electroplating, polymer resin content in paints and lacquers changing thus drastically the curing process etc.
Several methods for functionalizing the nanodiamonds with different functional groups have been developed. Typical functionalized nanodiamonds are carboxylated nanodiamonds, hydroxylated nanodiamonds and hydrogenated nanodiamonds, but contain still a mixture of typically oppositely charged functions and thus, mediocre zeta potential values and are thus not available in their solvent dispersion forms.
In prior art, methods, such as beads milling, to produce highly zeta negative, single digit carboxylated nanodiamond dispersions have been proposed. However, the methods have not been very successful.
It is known in the art that when subjecting a highly zeta negative, carboxylated nanodiamond powder to beads milling, a big part of existing surface carboxylic acid functions are subjected to reduction to zeta positive hydroxyl groups. This reduction is having an adverse effect on the material overall zeta potential, and results in severe agglomeration. Eventually the agglomerates clog the beads milling apparatus.
Publication A. Krueger and D. Lang, Adv. Funct. Mater. 2012, 22, 890-906 discloses the phenomena where nanodiamond particles, subjected to a beads assisted sonic disintegration (a method combining beads milling and ultrasonication), become significantly more hydrophilic, zeta potential rises to ˜+40 mV (at neural pH) and colloidal solutions of OH terminated nanodiamond particles are obtained.
Based on above disclosure, there is a qualitative and quantitative need for an efficient method for producing highly zeta negative single digit carboxylated nanodiamond dispersion and a product thereof.